It happens to the best of us. The correct way to handle this is to have buyers return items to you first. Then you reship them to the correct buyers. This of course all hinges on both cooperating, and that all the extra shipping is worth it.
Unfortunately, if one doesn't want to cooperate, there is nothing you can do. And of course that one is the one that got the most expensive item. In that case, you still have to refund in full upon the return from the other buyer.
Easiest way to get items back is to have each request a return, you approve it with you paying shipping and ebay will email them return labels.
